One of the most common uses of the preprocessor is to include another file:

#include <stdio.h>int main(void){printf("Hello, world!\n");return0;}

The preprocessor replaces the line #include <stdio.h> with the text of the file 'stdio.h', which declares the printf()function among other things.

This can also be written using double quotes, e.g. #include "stdio.h". If the filename is enclosed within angle brackets, the file is searched for in the standard compiler include paths. If the filename is enclosed within double quotes, the search path is expanded to include the current source directory. C compilers and programming environments all have a facility which allows the programmer to define where include files can be found. This can be introduced through a command line flag, which can be parameterized using a makefile, so that a different set of include files can be swapped in for different operating systems, for instance.

By convention, include files are given a .h extension, and files not included by others are given a .c extension. However, there is no requirement that this be observed. Occasionally you will see files with other extensions included: files with a .def extension may denote files designed to be included multiple times, each time expanding the same repetitive content; #include "icon.xbm" is likely to refer to an XBM image file (which is at the same time a C source file).

Most compilers targeting Microsoft Windows implicitly define _WIN32.[1] This allows code, including preprocessor commands, to compile only when targeting Windows systems. A few compilers define WIN32 instead. For such compilers that do not implicitly define the _WIN32 macro, it can be specified on the compiler's command line, using -D_WIN32.

The example code tests if a macro __unix__ is defined. If it is, the file <unistd.h> is then included. Otherwise, it tests if a macro _WIN32 is defined instead. If it is, the file <windows.h> is then included.

A more complex #if example can use operators, for example something like:

#if !(defined __LP64__ || defined __LLP64__) || defined _WIN32 && !defined _WIN64// we are compiling for a 32-bit system#else// we are compiling for a 64-bit system#endif

There are two types of macros, object-like and function-like. Object-like macros do not take parameters; function-like macros do. The generic syntax for declaring an identifier as a macro of each type is, respectively:

The function-like macro declaration must not have any whitespace between the identifier and the first, opening, parenthesis. If whitespace is present, the macro will be interpreted as object-like with everything starting from the first parenthesis added to the token list.

A macro definition can be removed with "#undef":

#undef <identifier> // delete the macro

Whenever the identifier appears in the source code it is replaced with the replacement token list, which can be empty. For an identifier declared to be a function-like macro, it is only replaced when the following token is also a left parenthesis that begins the argument list of the macro invocation. The exact procedure followed for expansion of function-like macros with arguments is subtle.

Object-like macros were conventionally used as part of good programming practice to create symbolic names for constants, e.g.

#define PI 3.14159

... instead of hard-coding the numbers throughout the code. An alternative in both C and C++, especially in situations in which a pointer to the number is required, is to apply the const qualifier to a global variable. This causes the value to be stored in memory, instead of being substituted by the preprocessor.

An example of a function-like macro is:

#define RADTODEG(x) ((x) * 57.29578)

This defines a radians-to-degrees conversion which can be inserted in the code where required, i.e., RADTODEG(34). This is expanded in-place, so that repeated multiplication by the constant is not shown throughout the code. The macro here is written as all uppercase to emphasize that it is a macro, not a compiled function.

The second x is enclosed in its own pair of parentheses to avoid the possibility of incorrect order of operations when it is an expression instead of a single value. For example, the expression RADTODEG(r +1) expands correctly as ((r +1)*57.29578); without parentheses, (r +1*57.29578) gives precedence to the multiplication.

Similarly, the outer pair of parentheses maintain correct order of operation. For example, 1/ RADTODEG(r) expands to 1/((r)*57.29578); without parentheses, 1/(r)*57.29578 gives precedence to the division.

Certain symbols are required to be defined by an implementation during preprocessing. These include __FILE__ and __LINE__, predefined by the preprocessor itself, which expand into the current file and line number. For instance the following:

prints the value of x, preceded by the file and line number to the error stream, allowing quick access to which line the message was produced on. Note that the WHERESTR argument is concatenated with the string following it. The values of __FILE__ and __LINE__ can be manipulated with the #line directive. The #line directive determines the line number and the file name of the line below. E.g.:

#line 314 "pi.c"puts("line="#__LINE__ " file=" __FILE__);

generates the puts function:

puts("line=314 file=pi.c");

Source code debuggers refer also to the source position defined with __FILE__ and __LINE__. This allows source code debugging, when C is used as target language of a compiler, for a totally different language. The first C Standard specified that the macro __STDC__ be defined to 1 if the implementation conforms to the ISO Standard and 0 otherwise, and the macro __STDC_VERSION__ defined as a numeric literal specifying the version of the Standard supported by the implementation. Standard C++ compilers support the __cplusplus macro. Compilers running in non-standard mode must not set these macros, or must define others to signal the differences.

Other Standard macros include __DATE__, the current date, and __TIME__, the current time.

The second edition of the C Standard, C99, added support for __func__, which contains the name of the function definition within which it is contained, but because the preprocessor is agnostic to the grammar of C, this must be done in the compiler itself using a variable local to the function.

Macros that can take a varying number of arguments (variadic macros) are not allowed in C89, but were introduced by a number of compilers and standardised in C99. Variadic macros are particularly useful when writing wrappers to functions taking a variable number of parameters, such as printf, for example when logging warnings and errors.

One little-known usage pattern of the C preprocessor is known as X-Macros.[2][3][4] An X-Macro is a header file. Commonly these use the extension ".def" instead of the traditional ".h". This file contains a list of similar macro calls, which can be referred to as "component macros". The include file is then referenced repeatedly.

Many compilers define additional, non-standard macros, although these are often poorly documented. A common reference for these macros is the Pre-defined C/C++ Compiler Macros project, which lists "various pre-defined compiler macros that can be used to identify standards, compilers, operating systems, hardware architectures, and even basic run-time libraries at compile-time".

The #pragma directive is a compiler-specific directive, which compiler vendors may use for their own purposes. For instance, a #pragma is often used to allow suppression of specific error messages, manage heap and stack debugging and so on. A compiler with support for the OpenMP parallelization library can automatically parallelize a for loop with #pragma omp parallel for.

C99 introduced a few standard #pragma directives, taking the form #pragma STDC ..., which are used to control the floating-point implementation.

Many implementations do not support trigraphs or do not replace them by default.

Many implementations (including, e.g., the C compilers by GNU, Intel, Microsoft and IBM) provide a non-standard directive to print out a warning message in the output, but not stop the compilation process. A typical use is to warn about the usage of some old code, which is now deprecated and only included for compatibility reasons, e.g.:

(GNU, Intel and IBM)

#warning "Do not use ABC, which is deprecated. Use XYZ instead."

(Microsoft)

#pragma message("Do not use ABC, which is deprecated. Use XYZ instead.")

Some Unix preprocessors traditionally provided "assertions", which have little similarity to assertions used in programming.[5]

Since the C preprocessor can be invoked independently to process files other than those containing to-be-compiled source code, it can also be used as a "general purpose preprocessor" (GPP) for other types of text processing. One particularly notable example is the now-deprecated imake system.

GPP does work acceptably with most assembly languages. GNU mentions assembly as one of the target languages among C, C++ and Objective-C in the documentation of its implementation of the preprocessor. This requires that the assembler syntax not conflict with GPP syntax, which means no lines starting with # and that double quotes, which cpp interprets as string literals and thus ignores, don't have syntactical meaning other than that.

However, since the C preprocessor does not have features of some other preprocessors, such as recursive macros, selective expansion according to quoting, string evaluation in conditionals, and Turing completeness, it is very limited in comparison to a more general macro processor such as m4.